Re: [PATCH mm-new v7 4/5] mm: khugepaged: skip lazy-free folios

["David Hildenbrand (Arm)" <david@kernel.org>]

On 2/7/26 14:51, Lance Yang wrote:
> 
> 
> On 2026/2/7 16:34, Barry Song wrote:
>> On Sat, Feb 7, 2026 at 4:16 PM Vernon Yang <vernon2gm@gmail.com> wrote:
>>>
>>> From: Vernon Yang <yanglincheng@kylinos.cn>
>>>
>>> For example, create three task: hot1 -> cold -> hot2. After all three
>>> task are created, each allocate memory 128MB. the hot1/hot2 task
>>> continuously access 128 MB memory, while the cold task only accesses
>>> its memory briefly and then call madvise(MADV_FREE). However, khugepaged
>>> still prioritizes scanning the cold task and only scans the hot2 task
>>> after completing the scan of the cold task.
>>>
>>> And if we collapse with a lazyfree page, that content will never be none
>>> and the deferred shrinker cannot reclaim them.
>>>
>>> So if the user has explicitly informed us via MADV_FREE that this memory
>>> will be freed, it is appropriate for khugepaged to skip it only, thereby
>>> avoiding unnecessary scan and collapse operations to reducing CPU
>>> wastage.
>>>
>>> Here are the performance test results:
>>> (Throughput bigger is better, other smaller is better)
>>>
>>> Testing on x86_64 machine:
>>>
>>> | task hot2           | without patch | with patch    |  delta  |
>>> |---------------------|---------------|---------------|---------|
>>> | total accesses time |  3.14 sec     |  2.93 sec     | -6.69%  |
>>> | cycles per access   |  4.96         |  2.21         | -55.44% |
>>> | Throughput          |  104.38 M/sec |  111.89 M/sec | +7.19%  |
>>> | dTLB-load-misses    |  284814532    |  69597236     | -75.56% |
>>>
>>> Testing on qemu-system-x86_64 -enable-kvm:
>>>
>>> | task hot2           | without patch | with patch    |  delta  |
>>> |---------------------|---------------|---------------|---------|
>>> | total accesses time |  3.35 sec     |  2.96 sec     | -11.64% |
>>> | cycles per access   |  7.29         |  2.07         | -71.60% |
>>> | Throughput          |  97.67 M/sec  |  110.77 M/sec | +13.41% |
>>> | dTLB-load-misses    |  241600871    |  3216108      | -98.67% |
>>>
>>> Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
>>> Acked-by: David Hildenbrand (arm) <david@kernel.org>
>>> Reviewed-by: Lance Yang <lance.yang@linux.dev>
>>> ---
>>>   include/trace/events/huge_memory.h |  1 +
>>>   mm/khugepaged.c                    | 13 +++++++++++++
>>>   2 files changed, 14 insertions(+)
>>>
>>> diff --git a/include/trace/events/huge_memory.h b/include/trace/ 
>>> events/huge_memory.h
>>> index 384e29f6bef0..bcdc57eea270 100644
>>> --- a/include/trace/events/huge_memory.h
>>> +++ b/include/trace/events/huge_memory.h
>>> @@ -25,6 +25,7 @@
>>>          EM( SCAN_PAGE_LRU,              
>>> "page_not_in_lru")              \
>>>          EM( SCAN_PAGE_LOCK,             
>>> "page_locked")                  \
>>>          EM( SCAN_PAGE_ANON,             
>>> "page_not_anon")                \
>>> +       EM( SCAN_PAGE_LAZYFREE,         
>>> "page_lazyfree")                \
>>>          EM( SCAN_PAGE_COMPOUND,         
>>> "page_compound")                \
>>>          EM( SCAN_ANY_PROCESS,           
>>> "no_process_for_page")          \
>>>          EM( SCAN_VMA_NULL,              
>>> "vma_null")                     \
>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>> index 8b68ae3bc2c5..0d160e612e16 100644
>>> --- a/mm/khugepaged.c
>>> +++ b/mm/khugepaged.c
>>> @@ -46,6 +46,7 @@ enum scan_result {
>>>          SCAN_PAGE_LRU,
>>>          SCAN_PAGE_LOCK,
>>>          SCAN_PAGE_ANON,
>>> +       SCAN_PAGE_LAZYFREE,
>>>          SCAN_PAGE_COMPOUND,
>>>          SCAN_ANY_PROCESS,
>>>          SCAN_VMA_NULL,
>>> @@ -583,6 +584,12 @@ static enum scan_result 
>>> __collapse_huge_page_isolate(struct vm_area_struct *vma,
>>>                  folio = page_folio(page);
>>>                  VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio);
>>>
>>> +               if (cc->is_khugepaged && !pte_dirty(pteval) &&
>>> +                   folio_test_lazyfree(folio)) {
>>
>> We have two corner cases here:
> 
> Good catch!
> 
>>
>> 1. Even if a lazyfree folio is dirty, if the VMA has the VM_DROPPABLE 
>> flag,
>> a lazyfree folio may still be dropped, even when its PTE is dirty.

Good point!

> 
> Right. When the VMA has VM_DROPPABLE, we would drop the lazyfree folio
> regardless of whether it (or the PTE) is dirty in try_to_unmap_one().
> 
> So, IMHO, we could go with:
> 
> cc->is_khugepaged && folio_test_lazyfree(folio) &&
>      (!pte_dirty(pteval) || (vma->vm_flags & VM_DROPPABLE))

Hm. In a VM_DROPPABLE mapping all folios should be marked as lazy-free 
(see folio_add_new_anon_rmap()).

The new (collapse) folio will also be marked lazy (due to 
folio_add_new_anon_rmap()) free and can just get dropped any time.

So likely we should just not skip collapse for lazyfree folios in 
VM_DROPPABLE mappings?

if (cc->is_khugepaged && !(vma->vm_flags & VM_DROPPABLE) &&
     folio_test_lazyfree(folio) && !pte_dirty(pteval)) {
	...
}


-- 
Cheers,

David